An azimuth thruster system can be operated by mechanical architectures called Z-drive or L-drive. The Z-drive or L-drive may include a power source (e.g., a prime mover) for propeller thrust installed inside a hull, and power from the power source may be transmitted to the propeller through a drive train including at least one pair of bevel gears located inside a pod that is rotatably mounted to the hull. The Z-drive or L-drive may further include an azimuth controller for controlling an orientation (e.g., azimuth) of the propeller thrust by turning the pod about a vertical axis (e.g. an azimuthal axis) relative to the hull of the ship.
However, there may be drawbacks in azimuth thruster systems including the Z-drive or L-drive. In some examples where the mechanical architecture uses a single power drive train from a prime mover to a propeller, the torque delivered by the prime mover may be applied not only to the propeller shaft through bevel gear engagement inside the pod but also onto the pod. For example, the torque generated by the prime mover is initially transmitted vertically and then redirected horizontally through engagement of two bevel gears inside the pod. In this example, it may be difficult to maintain or change orientation of the pod without applying a supplementary torque that can counteract the torque inevitably imposed vertically onto the pod in an effort to drive the propeller using the prime mover fixed inside the hull.
A purpose of an independent azimuth control system in the Z-drive or L-drive systems may include counteracting this unwanted torque imposed onto the pod during maintaining constant azimuth and delivering azimuth control torque by overcoming this unwanted torque when a change of azimuth is demanded. In some examples, even when there is no need for azimuthal change of the pod, the azimuth control system may waste some energy in generating the counteracting torque that has a magnitude directly proportional to a magnitude of the torque required to drive the propeller, unless there is an extra braking system that can be engaged to keep the azimuth constant. This in turn implies that the azimuth control system may be continuously exposed to some mechanical stresses that can be built up internally during a propeller operation, for example, in a ring and pinion gear system utilized in the azimuth control system. The torque and stress burden may be distributed by employing multiple prime movers in an azimuth control system. However, it may be difficult to increase the agility in dynamic azimuth maneuvering of the pod because of limited available power and a high gear ratio involved in ring and pinion gear engagement in the azimuth control system.
In some examples, this architectural weakness of Z-drive and L-drive systems may make it rather unpractical to use a prime mover with a low rotational speed and high torque for driving the propeller shaft because a low speed/high torque prime mover would increase the burden of the azimuth control system. In some cases, a high speed and low torque prime mover may be preferred in which torque is usually amplified and the rotational speed is reduced accordingly inside the pod by a pair of bevel gears before it is applied to the propeller shaft. For example, a small bevel gear in diameter may be attached to the vertical shaft driven by the prime mover while a large bevel gear is used to drive the horizontal shaft connected to a propeller. This constraint may lead to a voluminous design of the pod in terms of the effective cross-sectional area of the pod normal to the thrust, tending to increase hydrodynamic drag caused by the pod.
In some examples, an electrical type prime mover is installed inside the pod to directly apply torque to the propeller, which may be under a risk of damaging the prime mover in an event of leakage. In this example, electrical power and various signals may be exchanged through a slip-ring which can cause frequent maintenance problems. In some cases, this architecture may require an independent azimuth control system to control the azimuth of the pod. In this case, it may be still difficult to achieve agile dynamic maneuvering of the pod azimuth because the moment of inertia of the whole pod assembly about the azimuthal axis can be large in comparison to the power available to the azimuth control system, due to the heavy prime mover installed inside the pod.